1. Field of the Invention
The present invention relates generally to semiconductor apparatuses and methods of fabricating the semiconductor apparatuses and, more particularly, to a semiconductor apparatus in which a crystallized thin film is formed on a substrate, and a method of fabricating such a semiconductor apparatus.
2. Description of the Background Art
A semiconductor apparatus in which a thin film made such as of silicon is epitaxially grown on a surface of a semiconductor substrate such as a silicon substrate has been conventionally known. FIG. 31 is a schematic diagram for use in explaining a method of fabricating such a conventional semiconductor apparatus. FIGS. 32-34 are cross-sectional views for use in explaining a process for fabricating the conventional semiconductor apparatus. FIG. 35 is a cross-sectional view of a semiconductor fabricating apparatus for use in a conventional thin film formation. A conventional semiconductor fabricating apparatus will now be described with reference to FIG. 35. This conventional semiconductor fabricating apparatus includes cylindrical reaction chambers 111 and 112, a susceptor 114 for supporting a single-crystalline silicon substrate 101, a lid 119 on which susceptor 114 is securely attached, an elevator 123 for moving lid 119 up and down, a gas supply unit 115 for supplying a silane gas, a nitrogen gas, a hydrogen gas or the like through a pipe 116 into reaction chamber 111 while regulating the flow of the gas, a vacuum pump 117 for exhausting reaction chamber 111 of gas or air through an exhaust pipe 118, a heater 120 for heating reaction chamber 111, a manifold 121 having an opening through which pipe 116 and exhaust pipe 118 run and which is used for accommodating susceptor 114 into reaction camber 111, and a frame 122 for fixedly placing manifold 121 and elevator 123 thereon.
By using the semiconductor fabricating apparatus thus structured, a thin film formation has been conventionally made in the following process.
With reference to FIGS. 31-35, a single-crystalline silicon substrate 101 is first cleaned with a solution of hydrofluoric acid (HF) under the condition of room temperature. A silicon oxide film (SiO.sub.2) formed on a surface of the single-crystal silicon substrate is thereby removed. Single crystalline silicon substrate 101 is then rinsed with water, sprayed with a nitrogen gas and then dried, resulting in clean single crystalline silicon substrate 101 (see FIG. 32).
Then, with a lid 119 of the semiconductor fabricating apparatus moving downward, single crystalline silicon substrate 101 is mounted on susceptor 114. The temperature of reaction chamber 111 is now kept at 350.degree. C. In this state, lifting elevator 123 allows susceptor 114 to enter into reaction chamber 111 and reaction chamber 111 is sealed off or closed airtightly by lid 119. Reaction chamber 111 is then exhausted of air through exhaust pipe 118 by pump 117.
A nitrogen gas (N.sub.2) is then supplied from gas supply unit 115 into reaction chamber 111. The inside of reaction chamber 111 is substituted with the supplied nitrogen gas (N.sub.2) and then maintained under a predetermined pressure (e.g., 10 Torr).
Since single crystalline silicon substrate 101 before introduced into reaction chamber 111 is at room temperature, single crystalline silicon substrate 101 is kept in reaction chamber 111 under a nitrogen gas (N.sub.2) atmosphere until the substrate temperature reaches 350.degree. C.
After the temperature of single crystalline silicon substrate 101 becomes uniformly 350.degree. C., the temperature in reaction chamber 111 is still increased to 620.degree. C. Single crystalline silicon substrate 101 is kept in reaction chamber 111 under the nitrogen gas (N.sub.2) atmosphere until the respective temperatures of reaction chamber 111 and single crystalline silicon substrate 101 become uniformly 620.degree. C. The surface of single crystalline silicon substrate 101 is oxidized by oxygen (O.sub.2) and steam (H.sub.2 O) remaining in reaction chamber 111 before the temperature of substrate 101 rises from 350.degree. C. to 620.degree. C. This results in formation of a silicon oxide film (SiO.sub.x) 102 on the surface of single crystalline silicon substrate 101 (see FIG. 33). The resultant silicon oxide film has a thickness of approximately 10-15.ANG..
After the respective temperatures of single crystalline silicon substrate 101 and reaction chamber 111 uniformly reach 620.degree. C., a silane gas (SiH.sub.4) is introduced from gas supply unit 115 into reaction chamber 111. This silane gas allows a silicon thin film 103 to grow on the surface of silicon oxide film 102 (see FIG. 34). After that, the interior of reaction chamber 111 is again substituted with a nitrogen gas (N.sub.2).
Finally, after a pressure in reaction chamber 111 is returned to atmospheric pressure by employing the nitrogen gas (N.sub.2), elevator 123 descends lid 119. Single-crystal silicon substrate 101 is then taken out from susceptor 114.
As described above, conventionally, the surface of single-crystal silicon substrate 101 is oxidized by oxygen (O.sub.2) and steam (H.sub.2 O) left in reaction chamber 111 during the period that the temperature of single-crystal silicon substrate 101 increases from 350.degree. C. to 620.degree. C. Accordingly, silicon oxide film (SiO.sub.x) 102 of approximately 10-15.ANG. in thickness is formed on the surface of single-crystal silicon substrate 101.
If silicon thin film 103 is grown on such silicon oxide film 102 by using a silane gas, however, a disadvantage arises that silicon thin film 103 becomes a polycrystal silicon thin film 103 and consequently no single-crystalline thin film is obtained. A method of substitution with hydrogen (H.sub.2) at a high temperature of approximately 1100.degree. C. has been proposed in order to remove the above silicon oxide film 102. This method, however, adversely affects elements such that a diffusion layer constituting a transistor broadly extends since the method is adapted at a considerably high temperature. FIG. 36 is a sectional view showing the structure of a three-dimensional device for describing the problem in a method of replacing a silicon oxide film 102 by hydrogen at a high temperature of about 1110.degree. C. as described above. With reference to FIG. 36, in the three-dimensional device, an insulator layer 206 having a contact hole is formed on a single-crystalline substrate 201. Then, a single-crystalline silicon layer 204 is formed so as to be electrically connected to single-crystalline semiconductor substrate 201 at the contact hole and extend on and along insulator layer 206. A first MOS transistor 202 is formed at a predetermined region on the main surface of single-crystalline semiconductor substrate 201 and a second MOS transistor 205 is formed at a predetermined region on the main surface of single-crystalline silicon layer 204. In the three-dimensional device having such a structure, a formation of single-crystalline silicon layer 204 requires a removal of a silicon oxide film (not shown) on the main surface of single-crystalline semiconductor substrate 201 with which single-crystalline silicon layer 204 contacts. In such a case, it is also possible to remove the silicon oxide film by a hydrogen replacement method requiring the above-described high temperature of 1100.degree. C. However, the hydrogen replacement method requiring a high temperature of about 1100.degree. C. involves a disadvantage that source/drain regions 203 constituting first MOS transistor 202 formed on the main surface of single-crystalline semiconductor substrate 201 are extended due to the high temperature. As a result, the transistor characteristics of first MOS transistor 202 vary.
Also, there is another disadvantage that since silicon oxide film 102 is as thick as 10-15.ANG. (see FIG. 33), a contact resistance between single-crystal silicon substrate 101 and polycrystal silicon thin film 103 increases.
As has been described above, conventionally, it has been difficult to form a single-crystalline thin film without providing a heat treatment of a high temperature (1100.degree. C.), i.e., a temperature not higher than 800.degree. C. It has been also difficult to reduce a contact resistance between polycrystal silicon thin film 103 and single-crystal silicon substrate 101.